1. Field of the Invention
This invention relates to a solid electrolyte type fuel cell and a method for producing the same.
2. Related Art Statement
Recently, fuel cells have been noted as power generating implements. The fuel cell is an implement capable of directly converting chemical energy included in a fuel to electric energy. As the fuel cell is free from limitation of Carnot's cycle, the cell is a very promising technique owing to its inherently high energy conversion efficiency, wide latitude of fuels to be used (naphtha, natural gas, methanol, coal reformed gas, heavy oil and the like), less public nuisance, and high electric power generating efficiency without being affected by scales of installations.
Particularly, as the solid electrolyte fuel cell (referred to as "SOFC", hereinafter) operates at high temperatures such as 1,000.degree. C., activity of electrodes is very high. Moreover, the SOFC has low polarization and relatively high output voltage without requiring any catalyst of an expensive noble metal such as platinum, so that energy conversion efficiency is much higher than those of the other fuel cells. Furthermore, the SOFC is stable and has long service life because all the constituent materials of the SOFC are solid.
The production technique for preparing SOFC must be capable of preparing thin and airtight solid electrolyte films as quickly as possible at low costs.
At present, stabilized zirconia and lanthanum series perovskite complexed oxides are the most promissing and usual materials, respectively, for constituting the solid electrolyte films and the air electrodes of the SOFC (Energy Sogo Kagaku 13-2, 1990).
Usually, production methods of the solid electrolyte films and the air electrodes are classified as dry processes and wet processes. Electrochemical vapor deposition (EVD) and thermal spray are typical of the dry processes, while tape casting, slip casting and extrusion molding, etc., are typical of the wet processes (Energy Sogo Kagaku 15-2, 1990).
If the production is effected by the so-called "gas phase process" such as chemical vapor deposition (CVD), or EVD, etc., the apparatus used for the process becomes large, and the surface area that can be treated and the speed of the treatment are untolerably small. Moreover, the running cost of the gas phase process is expensive, because zirconium chloride in admixture with a helium gas or steam water inadmixture with oxygen is used.
If the solid electrolyte film is formed by plasma thermal spray, the film-forming rate can be made large, the handling of the apparatus is simple, and relatively dense thin films can be formed. Therefore, plasma thermal spray has conventionally been used (Sunshine Journal 2, [1], 1981; and Energy Sogo Kagaku 13-2, 1990).
It has also publicly been known to form a solid electrolyte film by solid soluting cerium oxide or zirconium oxide with a metal oxide of an alkaline earth metal or a rare earth element, etc., to prepare a raw material for thermal spray, adjusting the particle size of the raw material, and plasma spraying the adjusted raw material of the thermal spray (Japanese Patent Application Laid-open Nos. 61-198,569 and 61-198,570).
Meanwhile, the solid electrolyte films formed by plasma thermal spray usually have a porosity of more than 5% reaching even up to 10%, so that they are not sufficiently dense as the solid electrolyte films for use in SOFC, and cracks and layer defects occur in the films formed by plasma thermal spray. As a result, leakage of a fuel occurs thus permitting permeation of hydrogen, and carbon monoxide, etc., through the solid electrolyte film at the time of operating the SOFC to decrease electromotive force of the SOFC per unit cell thereof and thereby lower the output of the SOFC and conversion efficiency of the fuel to electric power.
The inventors previously proposed a technique of at first plasma thermal spraying a solid electrolyte film on a surface of the air electrode, and then heat treating the film to densify the same (Japanese Patent Application Laid-open No. 4-115,469 filed on Sep. 4, 1990). However, the inventors have found out afterwards, that if the heat treatment temperature is sufficiently high, a highly resistive layer made of electrically insulative lanthanum zirconate La.sub.2 Zr.sub.2 O.sub.7, etc., is formed at the interface between the solid electrolyte film and the air electrode.
There is also known a method of forming a film made of a raw material for solid electrolyte on the air electrode by a wet process, and sintering the film to join the solid electrolyte film to the air electrode. However, if the heat treatment is effected at around 1,250.degree. C. for the purpose, a highly resistive layer consisting of electrically insulative lanthanum zirconate La.sub.2 Zr.sub.2 O.sub.7 is formed on the interface between the solid electrolyte and the air electrode, which increases the internal resistance of the SOFC to lower the output of the SOFC.
Recently, a proposal has been made wherein stabilized zirconia is used as a raw material for solid electrolyte film, and a perovskite series complexed oxide of a formula (La.sub.1-y Sr.sub.y).sub.1-x MO.sub.3 wherein y is 0.ltoreq.y.ltoreq.0.2, x is 0&lt;x.ltoreq.0.2, M is Mn or Co and the A site is stoichiometrically portionally defected is used as the raw material for the air electrode film (Japanese Patent application Laid-open No. 3-59,953).
The inventors newly studied using such a defected perovskite series complexed oxide as a raw material for a free-standing type air electrode tube of a bottomed tubular shape. As a result, the inventors have found out that such a compound has a remarkably high sintering property owing to its rich content of manganese or cobalt as compared with that of nondefected perovskite structure. Therefore, the inventors have also found out a new problem in that the sintering of the air electrode tube progresses during the steps of various heat treatments of the air electrode tube for forming other cell components to decrease the ability of transporting an oxidizing agent to the interface between the solid electrolyte film and the air electrode tube.